Nationwide communication system

ABSTRACT

A two-way communication system for communication between a system network and a mobile unit. The system network includes a plurality of base transmitters and base receivers included in the network. The base transmitters are divided into zonal assignments and broadcast in simulcast using multi-carrier modulation techniques. The system network controls the base transmitters to broadcast in simulcast during both systemwide and zonal time intervals. The system network dynamically alters zone boundaries to maximize information throughput. The preferred mobile unit includes a noise detector circuit to prevent unwanted transmissions. The system network further provides an adaptive registration feature for mobile units which controls the registration operations by the mobile units to maximize information throughput.

This is a division of application Ser. No. 07/97,918, filed Nov. 12,1992, now U.S. Pat. No. 5,590,403.

I. BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to methods and systems for providingtwo-way communication capability between a central network and a mobileunit over a relatively large area, and more particularly to such methodsand systems which allow for rapid communication of large messages andefficient use of system resources.

B. Description of the Related Art

Conventional two-way portable/mobile wireless messaging systems oftenprovide a variety of services to subscribers. Conventional messagingsystems in particular provide one-way services using store and forwardtechniques to mobile receivers carried by the subscriber. A fundamentalgoal of two-way messaging systems is to provide a network ofinterconnected transmitters and receivers which provides sufficienttransmitted signal strength and receive capability to uniformly cover ageographic region. Some conventional messaging systems provide themessage to the user on a small viewing screen on the mobile unit.

However, such conventional systems often suffer from problems associatedwith low system throughput, evidenced by slow message delivery andmessage size limitations and do not provide an acknowledgment featurewherein the mobile unit transmits an acknowledgment signal to the systemto acknowledge receipt of the message from the system. Generally, systemthroughput refers to the overall communication capability of a system asdefined by the total amount of message data from the system to themobile units transferred by the system during a given period of timedivided by the frequency bandwidth necessary to transmit the messagedata and may be measured in bits transferred per Hz. Further, suchconventional systems suffer from technical problems preventingconsistent wide area coverage and would require extremely wide portionsof valuable frequency bandwidth to achieve acceptable system throughputrates.

Simulcast technology in communication systems was originally developedto extend transmitter coverage beyond that which could be obtained froma single transmitter. Over time, however, simulcasting has evolved intoa technique capable of providing continuous coverage to a large area.

Generally, simulcast technology provides multiple transmitters,operating on substantially the same frequencies and transmitting thesame information positioned to cover extended areas. As shown in FIG. 1,transmitter 100 generally provides coverage over area A, D, and E,transmitter 102 generally provides coverage over area B, D, and E, andtransmitter 104 generally provides coverage over area C, E, and F. Insome cases, the coverage area of a first transmitter may be entirelyenclosed within the coverage area of another transmitter, such as inbuilding interiors and valleys. In areas where one (and only one)transmitter dominates (e.g., areas A, B, and C in FIG. 1), simulcast iseffective because the other transmitters do not significantly affectreceivers in those areas.

However, in "overlap" areas D, E, and F shown in FIG. 1, where thesignals from two or more transmitters are approximately equal, problemscan arise because destructive interference of signals occurs in theseoverlap areas such as areas D, E, and F. Destructive interference occurswhen the two signals are equal in magnitude and 180° out of phase andcompletely cancel each other. While there were some successes, reliabledesign procedures were not available.

Attempting to precisely synchronize the carrier frequencies of allsimulcast transmitters does not overcome the problem because points(i.e. nodes) at which destructive summing occurred persisted for longperiods of time. At such points, a mobile receiver can not receive thesimulcast signal.

Deliberately offsetting the carrier frequencies of adjacent;transmitters can ensure that destructive interference does not persistat one point for an extended period of time. The slight errors infrequency displayed by high quality reference oscillators (e.g., 20hertz errors in 100 MHz signals or a few parts in 10⁷) render deliberateoffsetting unnecessary. Further, merely offsetting the carrierfrequencies could not guarantee acceptable quality demodulation becauseproper alignment of the modulating signals in time is also required.

FIG. 2 displays the situation at, for example, point D in FIG. 1 whenmodulating waveforms are synchronized and includes coverage boundary 202from a first transmitter and a second transmitter coverage boundary 204from a second adjacent transmitter. An equi-signal boundary 200 existswhere the signals from the first and second transmitters haveapproximately equal signal strengths. A more realistic equi-signalboundary would take into account natural and man-made topography andpropagation conditions, and therefore would probably not be a straightline.

FIGS. 3 and 4 generally illustrate various signals as they may occur ator near the equi-signal boundary 200 as shown in FIG. 2. In particular,FIGS. 3 and 4 illustrate various aspects of modulation synchronizationand how altering transmission parameters may affect the synchronization.In general, there are at least three sources which cause the signalsfrom the first transmitter and the second transmitter to be out ofsynchronization: (1) timing shifts in the delivery of the modulatingwaveform to each of the transmitters; (2) timing shifts internal to eachtransmitter; and (3) timing shifts caused by propagation distances andanomalies. From the perspective of a receiver located in an overlaparea, these three sources of timing shifts combine to produce an overalltiming shifts between the received signals from the first and secondtransmitters. In current commercial practice, the summation of thesethree components results in time shifts of about 200 microseconds. Thetiming shift present in simulcast systems disadvantageously limits thebaud rate at which information may be transferred. In general, FIGS. 3and 4 will also illustrate how timing shifts prevents high baud ratetransmissions.

A time line representation of a signal 306 from a first transmitter isshown in FIG. 3(A) and a signal 308 from a second transmitter is shownin FIG. 3(B), both from the perspective of a receiver located in anoverlap area. Vertical dashed lines 300 represent baud intervals on thetime axis. As can be seen from FIGS. 3(A) and (B), the signals 306 and308 are frequency modulated between a high and a low frequency value andthe signals 306 and 308 are exactly in phase. As will be appreciated,the timing shift between signals 306 and 308 must be small when comparedto the baud interval shown in FIGS. 3(A) and (B) since signals 306 and308 are in synchronization. Of course, as the baud interval decreases,the timing shifts will likely cause signals 306 and 308 to be out ofsynchronization.

FIGS. 3(C), (D), and (E) show the summation of these two signals 306 and308 at an equi-signal boundary, such as boundary 200 in FIG. 2. FIG.3(C) shows a composite signal 310 indicating that the frequencyinformation remains unchanged, FIG. 3(D) shows a linear graph 312 of therelative phase difference caused by a slight carrier frequencydifference between the signals from the first transmitter and the secondtransmitter. FIG. 3(E) shows a composite amplitude signal 314. A noisethreshold is indicated by the horizontal dashed line 304 in FIG. 3(E).

Of interest, FIG. 3(E) shows the composite amplitude signal 314 dippingbelow the noise threshold 304 at an anti-phase condition 302 (e.g., whenthe relative phase angle is ±180°, as shown in FIG. 3(D)). As can beseen from FIG. 3(E), the anti-phase condition 302 caused by the slightphase shift between transmitter 1 and transmitter 2 will not cause anyloss of data because the anti-phase condition persists for only a smallportion of the baud interval.

The slight offset of the carrier frequencies between the first andsecond transmitters causes a slow drift of the relative phase of the twosignals, as shown in FIG. 3(D). When the signals are ±180° out of phase,the temporary dip in the amplitude signal may cause the loss of a fewbits in the composite signal, at worst. These errors can be counteractedwith a conventional error correcting code, such as is commonly known.

FIG. 4 shows a set of similar signals to those in FIG. 3, but whereinthe signal 402 from the first transmitter is offset from, or out ofsynchronization with, the signal 404 from the second transmitter by afull baud. In particular, signal 404 lags signal 402 by one baudinterval. As previously discussed, the offset of signals 402 and 404 maybe caused by various timing shifts in the delivery of both signals 402and 404 to a receiver in an overlap area. FIGS. 4(A) and (B) illustratethe extreme case where the sum of these timing shifts is equal to thebaud interval shown by dashed lines 400. As can be seen in FIG. 4(C),composite signal 406 includes a period of indeterminate frequency whichundesirably covers several entire baud intervals and, therefore,successful demodulation is impossible during those baud intervals. Ifthe baud interval were increased to minimize the effect of these timingshifts, data loss would be less likely. Therefore, it can be seen thatthe baud rate at which good data transfer can be accomplished is limitedby the timing shifts between signals delivered to receivers in overlapareas.

Through these examples, it can be seen that high degrees of modulationsynchronization make it possible to obtain good data demodulation in asimulcast system. However, the baud rate limitation of simulcast systemsis a significant drawback and limits system throughput.

An alternative to simulcast for wide area coverage is assignment oforthogonal, non-overlapping subdivisions of the available systemcapacity to adjacent areas. Subdivisions can be made in time (e.g.,broadcasting the information on the same frequency in different timeslots to adjacent areas), or in frequency (e.g., broadcasting theinformation simultaneously on different frequencies in adjacent areas).There are several problems with such orthogonal systems, however. First,orthogonal assignments require tuning the receiver to the assignedfrequency or time channel for the area in which the receiver currentlyresides. In the broadcast services every traveler has experienced thefrustration of finding the correct channel for their favorite programs.Simulcast operation avoids the need for scanning and re-tuning as themobile unit moves between areas. Such scanning and re-tuning alsodisadvantageously increases mobile unit power consumption.

Second, and more serious, the orthogonal assignment approach drasticallyreduces the system throughput capacity as measured in bits per Hzbecause anywhere from 3 to 7, or possibly more, orthogonal assignmentsare required to obtain continuous area coverage in most conventionalorthogonal systems. This waste of capacity is somewhat recouped if thesame information is not needed throughout the service area because agiven piece of information is sent only to those cells where it isneeded.

Conventional cellular radio service is a typical example of anorthogonal system. In cellular, the same frequencies are reused inspatially separated cells to allow different data to be transmitted todifferent mobile units. An example of three cellular arrangements isshown in FIG. 5 where the number of cells (N) is equal to 3, 4, and 7.Each cell (i.e., A, B, C, . . . ) in conventional cellular serviceusually only includes a single transmitter and operates in a differentfrequency or time division within the communication protocol. As shownin FIG. 5, cellular service generally locates transmitters utilizing thesame division (all the "A" transmitters) far enough apart to reduce thelikelihood of interference between such transmitters. As the number ofcells increases, the likelihood of interference decreases. For example,with N=3 as shown by arrangement 500 in FIG. 3, the distance between thecoverage area of "A" cells is about 1/2 cell width, with N=4 inarrangement 502, the distance between the coverage areas of "A" cells isslightly larger, and with N=7 in arrangement 504 the distance between"A" cells is larger than the width of one cell.

However, as the number of cells increases, the length of the individualtime intervals per cell decreases for time division multiplexed systems,thereby decreasing the systems total information transfer. In frequencydivision systems, more cells undesirably increases the frequencybandwidth required. Therefore, system throughput in bits per Hz isdecreased as the number of cells increases. Furthermore, cellularsystems often require an electronic "handshake" between system andmobile unit to identify the specific cell (i.e. transmitter) in whichthe mobile unit is located to allow capacity reuse.

II. SUMMARY OF THE INVENTION

The systems and methods of the present invention have a wide variety ofobjects and advantages. The systems and methods of the present inventionhave as a primary object to provide a communication system with widearea coverage and high message throughput while minimizing frequencybandwidth usage.

It is an object of the invention to provide a simulcast communicationsystem with a high data transfer rate which does not exceed the baudrate limitations of simulcast transmission.

It is a further object of the present invention to provide acommunication system which provides for superior data communicationintegrity.

Yet another object of the invention is to provide a mobile transceiverunit which prevents unnecessary RF interference, particularly oncommercial aircraft.

Still further, it is an object of the invention to provide a zone basedcommunication system which may dynamically redefine zone boundaries toimprove information throughput.

Another object of the invention is to provide a zone based simulcastcommunication system which can effectively communicate with both mobiletransceiver units located near the center of each zone as well as mobiletransceiver units located within the overlap areas between two or morezones.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practicing the invention. Theobjects and advantages of the invention will be realized and attained bymeans of the elements and combinations particularly pointed out in theappended claims.

To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the invention isdirected to a method for information transmission by a plurality oftransmitters to provide broad communication capability over a region ofspace, the information transmission occurring during at least both afirst time period and a second time period and the plurality oftransmitters being divided into at least a first and second set oftransmitters, the method comprising the steps of (a) generating a systeminformation signal which includes a plurality of blocks of information,(b) transmitting the system information signal to the plurality oftransmitters, (c) transmitting by the first and second sets oftransmitters a first block of information in simulcast during the firsttime period, (d) transmitting by the first set of transmitters a secondblock of information during the second time period, and (e) transmittingby the second set of transmitters a third block of information duringthe second time period.

In another embodiment, the invention is directed to a multi-carriersimulcast transmission system for transmitting in a desired frequencyband a message contained in an information signal, the system comprisinga first transmitter means for transmitting an information signal bygenerating a first plurality of carrier signals within the desiredfrequency band and by modulating the first plurality of carrier signalsto convey the information signal, and a second transmitter means,spatially separated from the first transmitter, for transmitting theinformation signal in simulcast with the first transmitter by generatinga second plurality of carrier signals at substantially the samefrequencies as the first plurality of carrier signals and by modulatingthe second plurality of carrier signals to convey the informationsignal.

In another embodiment, the invention is directed to a communicationmethod implemented in a computer controlled communication network forlocating a mobile transceiver within a region of space, the region ofspace being divided into a plurality of zones with each zone serviced byat least one base transmitter and at least one base receiver, thenetwork storing data corresponding to a zone where the mobiletransceiver was last known to be located, the communication methodcomprising the steps of (a) transmitting a message signal by a basetransmitter servicing a zone where the mobile transceiver was last knownto be located, (b) transmitting a systemwide probe signal by a pluralityof base transmitters servicing a plurality of zones if the mobiletransceiver does not indicate receipt of the message signal from thebase transmitter, (c) receiving the regional probe signal by the mobiletransceiver, (d) transmitting an acknowledgment signal by the mobiletransceiver in response to the received regional probe signal, (e)receiving the acknowledgment signal from the mobile transceiver by abase receiver, and (f) updating the data to reflect the zone of the basereceiver that received the acknowledgment signal as the last knownlocation of the mobile transceiver.

In yet another embodiment, the invention is directed to a method ofcommunicating messages between a plurality of base transmitters andmobile receivers within a region of space divided into a plurality ofzones with each zone having at least one base transmitter assignedthereto, the communication method comprising the steps of (a)transmitting substantially simultaneously a first information signal anda second information signal to communicate messages to the mobilereceivers, the first information signal being transmitted in simulcastby a first set of base transmitters assigned to a first zone, and thesecond information signal being transmitted in simulcast by a second setof base transmitters assigned to a second zone, (b) dynamicallyreassigning one or more of the base transmitters in the first set ofbase transmitters assigned to the first zone to the second set of basetransmitters assigned to the second zone as a function of the messagesto be communicated in an area, thereby creating an updated first set ofbase transmitters and an updated second set of base transmitters, and(c) transmitting substantially simultaneously a third information signaland a fourth information signal, the third information signal beingtransmitted in simulcast by the updated first set of base transmitters,and the fourth information signal being transmitted in simulcast by theupdated second set of base transmitters to communicate additionalmessages to said mobile receivers.

In another embodiment, the invention is directed to a mobile transceiverunit for transmitting messages to and receiving messages from a networkcomprising input means for allowing the user to input a user message tothe unit, transmitter means for transmitting a radio frequency signalincluding the user message from the mobile unit to the network, receivermeans for receiving radio frequency signals having a message from thenetwork, signal, detector means for detecting at least one type ofelectromagnetic signal generated external to the mobile unit and thenetwork, and a circuit, connecting the signal detector means to thetransmitter means, for disabling the transmitter means upon detection ofthe electromagnetic signal, thereby preventing unwanted radio frequencytransmission.

In another embodiment, the invention is directed to a communicationmethod for controlling a mobile transceiver which may communicate with acommunication network controlled by a computer, the network including aplurality of base transmitters for transmitting messages from thenetwork to the mobile transceiver and base receivers for receivingmessages from the mobile transceiver, the mobile transceiver beingcapable of sending a registration signal to be received by a basereceiver in the network to identify the mobile transceiver's locationand the plurality of base transmitters in the network being capable ofsending a probe signal to the mobile transceiver to cause the mobiletransceiver to transmit a signal to a base receiver to identify itslocation, the method comprising the steps of (a) sending a message fromthe network to the mobile transceiver to disable the mobiletransceiver's capability to transmit a registration signal, (b) storingthe number of probe signals sent by the network to the mobiletransceiver during a first period of time and the number of messagessuccessfully delivered to the mobile transceiver by the network during asecond period of time, (c) processing by the computer the stored numberof probe signals and number of messages successfully delivered toevaluate a likelihood that a probe signal will be required to be sent bythe network to locate the mobile unit to deliver a message, and (d)sending a message to the mobile unit to enable the mobile transceiver'scapability to transmit a registration signal if the calculatedlikelihood exceeds a selected value.

Finally, in another embodiment, the invention is directed to acommunication method for controlling a mobile transceiver which maycommunicate with a communication network controlled by a computer, thenetwork including a plurality of base transmitters for transmittingmessages to the mobile transceiver and base receivers for receivingmessages from the mobile transceiver, the mobile transceiver beingcapable of sending a registration signal to be received by a basereceiver in the network to identify the mobile transceiver's location,the network using received registration signals to determine a set ofbase transmitters to be operated to transmit a message to the mobiletransceiver, the method comprising the steps of (a) sending a messagefrom the network to the mobile transceiver to enable the mobiletransceiver's capability to transmit a registration signal, (b) storingthe number of registration signals from the mobile transceiver to thenetwork during a first period of time and the number of messagessuccessfully delivered to the mobile transceiver by the network during aperiod of time, (c) processing the stored number of registration signalsand number of messages successfully delivered to evaluate a likelihoodthat a registration signal from said mobile unit will not be used by thenetwork to determine a set of base transmitters, and (d) sending amessage to the mobile unit to disable the mobile transceiver'scapability to transmit a registration signal if the likelihood exceeds aselected value.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

III. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic diagram of an arrangement of simulcasttransmitters;

FIG. 2 is a schematic diagram of uniform smooth earth propagation;

FIG. 3 is a schematic diagram of synchronized modulated waveforms;

FIG. 4 is a schematic diagram of modulated waveforms offset a full baud;

FIG. 5 is a schematic diagram of cellular system coverage;

FIG. 6 is a schematic diagram of a communication system;

FIG. 7 is a flow chart of a preferred method of communication;

FIG. 8 is a flow chart of a preferred method of sending a regional probesignal;

FIG. 9 is a schematic diagram of a frequency spectrum for multi-carriermodulation;

FIG. 10 is a schematic diagram of an on/off keying modulator;

FIG. 11 is a schematic diagram of a frequency shift keying modulator;

FIG. 12 is a schematic diagram of a four carrier quadrature modulator;

FIG. 13 is a schematic diagram of a first embodiment of a basetransmitter;

FIG. 14 is a schematic diagram of a second embodiment of a basetransmitter;

FIG. 15 is a schematic diagram of a mobile transceiver;

FIG. 16 is a pictorial representation of a mobile transceiver;

FIG. 17 is a schematic diagram of a mobile receiver;

FIG. 18(A) is a schematic diagram of an analog base receiver;

FIG. 18(B) is a schematic diagram of a digital base receiver;

FIG. 19 is a schematic diagram of a base receiver with a store andforward feature;

FIG. 20 is a schematic diagram of a network operations center;

FIG. 21 is a schematic diagram of a database structure;

FIG. 22 is a schematic diagram of a traffic database;

FIG. 23 is a schematic diagram of a service queue;

FIG. 24 is a schematic diagram of a base transmitter database;

FIG. 25 is a schematic diagram of dynamically changing zonalassignments;

FIG. 26 is a flow chart of a preferred method of dynamically zonalreassignment;

FIG. 27(A) is a schematic diagram of the cycle protocol;

FIG. 27(B) is a schematic diagram of the forward batch intervalprotocol;

FIG. 27(C) is a schematic diagram of the individual batch protocol;

FIG. 28(A) is a flow chart of a preferred method to enable theregistration feature of a mobile unit;

FIG. 28(B) is a flow chart of a preferred method to disable theregistration feature of a mobile unit;

FIG. 29(A) is a flow chart of a preferred evaluation method used toenable the registration feature; and

FIG. 29(B) is a flow chart of a preferred method used to disable theregistration feature.

IV. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments and exemplary embodiments of the invention, examples ofwhich are illustrated in the accompanying drawings. Wherever possible,the same reference numbers will be used throughout the drawings to referto the same or like parts.

A. Overview of the System Hardware

FIG. 6 shows an overview of the major elements of a preferredcommunication system according to the present invention. As showntherein, the communication system includes a network operations center600 which is connected to a satellite uplink 602 via data path 604. Asatellite uplink is used to provide data to satellite 606. Satellite 606redirects the received data to several satellite downlink stationsincluding station 608 and station 610. Conventional satellite technologyallows for nominal data transfer rates of 24M bits/second. Further,conventional satellite technology allows for accurate delivery of datato stations 608 and 610, which allows for precise synchronizationbetween the signals broadcast in simulcast by the stations 608 and 610.It should be understood that stations 608 and 610 may optionally receiveidentical data, or may individually receive different datasimultaneously from the satellite 606.

Satellite downlink stations 608 and 610 are connected to spatiallyseparated base transmitters 612 and 614 via data paths 616 and 618,respectively. Base transmitter 612 is connected to antenna 620, and basetransmitter 614 is connected to antenna 622. Preferably, the basetransmitters of the present system have a power output capability ofabout 350 watts, which will provide an effective transmitter coveragearea of several tens of miles. Although not shown in FIG. 6, each zonepreferably includes multiple transmitter stations as will be evidentfrom the following discussion.

Mobile unit 624 is connected to antenna 626 and, in the preferredembodiment, is a small, portable unit capable of being carried easily bya user and therefore is similar to conventional pagers in those aspects.More preferably, the mobile unit has both receive and transmitcapability, with a nominal transmit power output of about 1 watt.

The communication system includes several base receivers 628, 630, 632,and 634 each connected to antennas 636, 638, 640, and 642, respectively.Base receivers 628 and 630 are connected to a regional station 644 viadata paths 646 and 648, respectively. Base receivers 632 and 634 areconnected to regional station 650 via data paths 652 and 654,respectively. Base transmitters 612, 614 preferably have a largetransmit power output capability to provide coverage to the mobile unitin areas to which communication is typically difficult, such as buildinginteriors, and to extend the coverage area of each transmitter. Anappropriate number of base receivers should be dispersed throughout thegeographic area to reliably receive the signals from the mobile unit.Due to the difference in output power between base transmitters andmobile units, an overall ratio of 10 base receivers to 1 basetransmitter may be appropriate, and the 2 to 1 ratio shown in FIG. 6 ismerely shown for ease of illustration.

Regional station 650 is connected to the network operations center 600via data path 656 and regional station 644 is connected to the networkoperations center 600 via data path 658. The data paths 656 and 658preferably include low cost phone lines, but may include any convenientand appropriate data transfer technology.

Generally, the communication system of the present invention roughlydivides various regions of space into portions called zones. Each zonemust have one or preferably more base transmitters assigned to it. Zoneboundaries are roughly defined by the transmitter coverage areas of thebase transmitters assigned to that zone. For example, FIG. 6 shows adashed zone dividing line 660 roughly dividing a zone 1 from a zone 2Zone 1 includes base transmitter 614, base receivers 632 and 634,regional station 650, and mobile unit 624. Zone 2 includes basetransmitter 612, base receivers 628 and 630, and regional station 644.Dashed line 660 only roughly defines the boundary between zones becauseprecise boundaries do not exist. For example, to insure adequatecoverage of the region, as shown in FIG. 1, the range of bothtransmitter 614 should at least cover the region above dashed line 660,and preferably should extend somewhat below dashed line 660. Similarly,the range of base transmitter 612 should at least cover the region belowdashed line 660, and preferably should extend somewhat above dashed line660. As can be seen, an overlap of transmitter coverage may occur in thevicinity of dashed line 660.

Referring back to FIG. 2, it can be seen that boundary 202 and boundary204 overlap in an area near the equi-signal 200 and between theseboundaries which may be termed an "overlap area." In FIG. 6, dashed line660 is drawn near the may be defined as the equi-signal boundary betweenbase transmitter 614 and base transmitter 612. Of course, dashed line660 does not represent the overlap area that may occur between basetransmitter 614 and base transmitter 612.

As explained in the Background of the Invention section, if basetransmitters 612 and 614 are broadcasting identical signals ton the samefrequencies in simulcast, good reception by a receiver located near thedashed line 660, and possibly in an overlap area (not shown), can beachieved. Simulcast thus may provide uniform transmitter coverage forthe region shown in FIG. 6. However, if base transmitter 612 isbroadcasting a first information signal and base transmitter 614 isbroadcasting a different, second information signal on identicalfrequencies simultaneously, it will likely be difficult for a receiverlocated in the overlap area to receive either the first or the secondinformation signal. In this instance, the overlap area may be referredto as an interference area because a receiver in this area would receivea composite signal, including the first and second information signal,that would likely be unusable.

The following will be an exemplary discussion of the variousinteractions of the elements of the communication system when deliveringa message to mobile unit 624. In accordance with the invention, apreferred method 700 of this interaction is shown in FIG. 7. Networkoperations center 600 generates a system information signal of severalblocks of information as shown in step 702. The blocks of informationinclude an electronic message to be delivered to the mobile unit 624.

In step 704, the system information signal is transmitted to the basetransmitters. In particular the network operations center 600 providethe system information signal and appropriate other data to thesatellite uplink 602 via data path 604 for transmission to the satellite606. The data is then received and retransmitted by satellite 606 tosatellite downlink stations 608 and 610. The data received by satellitedownlink 608 is provided to base transmitter 612 through data path 616,and the data received by satellite downlink 610 is provided to basetransmitter 614 through data path 618.

At this point, the exemplary communication system shown in FIG. 6 maytransfer the message to the mobile unit during one of two timeintervals. In the first time interval, both base transmitter 612 andbase transmitter 614 transmit data via antenna 620 and antenna 622,respectively, in simulcast to be received by mobile unit 624, whichcorresponds to step 706 in FIG. 7. This first alternative may be usefulto deliver the message if, for example, the location of mobile unit 624in zone 1 or zone 2 is unknown and broad coverage is desired.

In the second time interval, base transmitter 614 transmits a block ofinformation including the message data to mobile unit 624 and basetransmitter 612 transmits another block of information, whichcorresponds to steps 708 and 710 of FIG. 7. This second alternative maybe useful if, for example, the mobile unit 624 is known to be located inzone 1 and out of range of base transmitter 612. Delivery of the messageto mobile unit 624 during the second time interval is advantageousbecause during message delivery to the mobile unit 624 by basetransmitter 614, base transmitter 612 could be delivering a differentmessage to a different mobile unit (not shown). As can be seen, thissecond alternative would increase information throughput and systemefficiency.

If the mobile unit 624 has properly received the message via antenna626, then the mobile unit 624 may generate a return signal and broadcastthat signal via antenna 626 The return signal may be received by any orseveral of the base receivers 628, 630, 632, or 634. For example, thereturn signal could be received by base receiver 632 through antenna 640if antenna 640 is located closer to the mobile units than any otherantenna 636, 638, or 642. In this case, the base receiver would receivethe return signal and provide it to regional station 650 through datapath 652. The regional station would then provide the return signal tothe network operations center 600 through data path 656 for furtherprocessing as appropriate. It should be understood that a return signalmay include either an autonomous acknowledgment signal which indicatesthat the mobile unit accurately received the message or a user generatedreply signal.

If the mobile unit 624 does not completely receive the message, it cangenerate and broadcast a negative acknowledge signal. The negativeacknowledge signals when delivered to the network operations center 600,indicates that retransmission of the message is necessary.

It should be understood that the exemplary system shown in FIG. 6includes a modest number of elements for ease of explanation. It isenvisioned that the system of the present invention include a largenumber of base transmitters, base receivers, regional stations, andmobile units with a substantial number of base transmitters assigned toeach zone and all base transmitters assigned to a particular zoneoperating in simulcast. Further, it is envisioned that the presentsystem could advantageously support a large number of zones to cover awide geographic area.

B. Overview of the Zonal Simulcast Concepts

The preferred systems and methods of the present invention variously usesimulcast techniques within individual zones and over several or all ofthe zones. As previously noted, zones are generally defined by thecoverage areas of the one or more base transmitters. The networkoperations center 600 assigns each base transmitter in the system to azone. For example, in FIG. 6, base transmitter 614 is assigned to zone1, and the base transmitter 612 is assigned to zone 2 by the networkoperations center 600. To maximize information throughput, the systemsand methods of the present invention dynamically control zonalassignments and the use of simulcast techniques.

In general, the communication system of the present invention operatesby repeating a communication cycle to achieve desired informationtransfer, which is more fully discussed infra. The communication cycleis divided into a systemwide time interval and a zonal time interval. Inthe systemwide time interval, the base transmitters from at leastseveral zones are operated in simulcast to simultaneously transmitidentical information to a large geographic area. It should beunderstood that the systemwide time merely two or more zones.

Broadly speaking, the communication system need not know the location ofa mobile unit to transmit to it during the systemwide time interval.Therefore, the systemwide time interval can be used to send a "probe"signal that requests a particular mobile unit to broadcast anacknowledgment signal to allow the system to determine its approximatelocation by determining which base receiver receives the acknowledgmentsignal. Probe signals, thereby, may be used to track the locations ofmobile units, or to uncover the location of "lost" mobile units.

In the zonal time interval, each base transmitter assigned to aparticular zone transmits identical information in simulcast. However,for mobile units at or near the interference areas between adjacentzones, poor communication to those mobile units is likely during thezonal time interval because transmitters in adjacent zones will besimultaneously transmitting different data on the same, or substantiallythe same, frequencies. The zonal time interval provides goodcommunication capability for mobile units not located near the zonalboundaries and allows the system to "reuse" identical frequencies inadjacent zones. Furthermore, if zonal boundaries are selected to belocated in areas where mobile units are not likely to be located, i.e.unpopulated areas, the likelihood of providing good communicationcapabilities to a large percentage of mobile units can be increased.

As can be seen, from a system perspective, it is desirable tocommunicate with the mobile units in the zonal time interval becauseinformation throughput is maximized by reusing the transmissionfrequency band in the several zones. In other words, using the zonaltime interval allows communication with a large number of mobile unitsin a short amount of time. Accordingly, communication during thesystemwide time interval should be minimized because messagetransmission during this interval requires a large amount of systemresources be dedicated to that message.

For mobile units located near the boundaries between zones whereinterference is likely during the zonal time interval, goodcommunication capability can be achieved for these units during thesystemwide time interval. In the preferred systems and methods, when amobile unit fails to acknowledge a message sent during the zonal timeinterval or provides a negative acknowledgment, the network operationscenter sends a probe signal during a subsequent systemwide time intervalto determine the location of that mobile unit. If the location of themobile unit indicates that a likely reason for the failure of the mobileunit to receive the message is caused by inter-zonal interference, thenetwork operations center may simply retransmit the message during thesystemwide time interval. In other instances, the failure tosuccessfully deliver a message may be simply caused by the mobile unitbeing located in a weak signal area within a zone. In these instances,the system may retransmit the message during the zonal time intervalusing an appropriate error correcting code or using a stronger errorcorrecting code.

Alternatively, the network operations center may determine from theprobe signal that the mobile unit is simply located in a different zonethan the zone that the message was first sent. In this case, the networkoperations center preferably causes the message to be retransmitted inthe appropriate zone without again using a portion of the valuablesystemwide time interval.

In accordance with the invention, a preferred method 800 for sending aprobe signal is shown in FIG. 8. In step 802, a message signal istransmitted by a base transmitter servicing a zone where the mobiletransceiver was last known to be located. In particular, this may bepreferably an attempt by the network to deliver a message to the mobiletransceiver.

If the mobile transceiver does not indicate receipt of the messagesignal from the base transmitter transmitted in step 802, the networkassumes that the mobile transceiver has not received the message andtransmits a probe signal by a plurality of base transmitters servicing aplurality of zones in step 804. The mobile transceiver receives theprobe signal in step 806.

Upon receipt of the probe signal by the mobile transceiver, the mobiletransceiver transmits an acknowledgment signal in step 808. A basereceiver receives the acknowledgment signal from the mobile transceiverin step 810.

Finally, the data, such as the last location field 2104 shown in userdatabase 2100, is updated to reflect the zone of the base receiver, orreceivers, that receives the acknowledgment signal as the last knownlocation of the mobile transceiver in step 812.

C. The Multi-Carrier Modulation Transmission Format

The base transmitters of the communication system, such as basetransmitters 612 and 614 shown in FIG. 6, preferably utilize amulti-carrier modulation format as will now be described. In general, amulti-carrier modulation format envisions the simultaneous transmissionof several closely spaced carrier frequencies within a desired frequencyband, each individually modulated to convey an information signal. Themulti-carrier modulation format advantageously allows for high datatransfer rates by providing good bit rate transmission rates whilekeeping below the baud rate limitations of simulcast transmissiontechniques.

FIG. 9 shows a frequency representation 900 of an eight carriermodulation format. Carrier frequency 902 is shown with side bands 904,carrier frequency 906 is shown with side bands 908, carrier frequency910 is shown with side bands 912, carrier frequency 914 is shown withside bands 916, carrier frequency 918 is shown with side bands 920,carrier frequency 922 is shown with side bands 924, carrier frequency926 is shown with side bands 928, and carrier frequency 930 is shownwith side bands 932.

It should be understood that although this exemplary figure shows aneight carrier signal modulation format, other different numbers ofcarrier frequencies may be considered for use in the systems and methodsof the present invention.

In this exemplary embodiment, the carrier frequencies are spaced 3 KHzapart within a desired frequency band of 50 KHz. Dashed line skirts 934and 936 represent minimum frequency roll off levels, such as may berequired by Federal Communication Commission regulations, to preventoverlap interference into adjacent frequency bands.

Because eight unique data streams may be modulated onto the respectiveeight carrier signals in this embodiment, the data transfer rate of thetransmission from the base transmitters can be greatly increased, whilekeeping the baud rate within acceptable ranges for simulcasttransmission. It should also be understood that in accordance with goodsimulcast practice, the respective carrier frequencies between adjacentbase transmitters, such as base transmitter 612 and base transmitter 614in FIG. 6, should be slightly offset to prevent sustained nodes or "deadspots" where destructive interference between the signals from eachtransmitter provides an unusable composite signal, as was explained inthe background section of this application. This frequency offset ispreferably on the order of 10-20 hertz.

As previously discussed, each carrier signal may be individuallymodulated to convey a data stream. The following will discussalternative techniques for modulating a plurality of carriers inaccordance with the systems and methods of the present invention.

1. Modulated On/Off Keying

Perhaps the simplest modulation scheme conceptually is modulated on/offkeying (MOOK). FIG. 10 shows a schematic representation of a MOOKmodulator 1000. The MOOK modulator 1000 includes a plurality of carrierfrequency generating devices, such as frequency generator 1002generating frequency F1, frequency generator 1004 generating frequencyF2, frequency generator 1006 generating frequency F3, frequencygenerator 1008 generating frequency F4, and frequency generator 1010generating frequency Fn. As shown in FIG. 10, the MOOK modulator 1000may include any number (i.e. n) of frequency generators, but eightcarrier frequencies are preferred, as shown in FIG. 9.

The output from each of the carrier frequency generators 102, 104, 106,108, and 110 is applied to a plurality of respective switches SW1 812,SW2 814, SW3 816, SW4 818, and SWn 820. The output from each switch isprovided to a combiner 1022.

Each of the switches SW1 812, SW2 814, SW3 816, SW4 818, and SWn 820opens and closes under the control of a control logic system (not shown)to effect the MOOK modulation. The control logic system (not shown)causes the desired switches to variously close and open, therebyconveying an n-bit binary word. Each carrier frequency transmits abinary "one" if the respective switch is closed and a binary "zero" ifthe respective switch is open.

The summer 1022 combines the modulated carrier frequencies to provide amulti-carrier modulated output signal that conveys an n-bit binary word.

2. Binary Frequency Shift Keying Modulation

An alternative multi-carrier modulation scheme including frequency shiftkeying (FSK) techniques may be implemented by the modulator shown inFIG. 11. A frequency shift keying modulator 1100 includes a firstfrequency source 1102, a second frequency source 1104, a third frequencysource 1106, a fourth frequency source 1108, and an nth frequency source1110. The output from each frequency source is provided to a respectivemodulator 1112, 1114, 1116, 1118, and 1120.

A control logic system (not shown) provides a frequency control signalto each modulator to frequency shift modulate the carrier frequencies.In particular, the control logic system (not shown) provides frequencycontrol signal 1 to modulator 1112, frequency control signal 2 tomodulator 1114, frequency control signal 3 to modulator 1116, frequencysignal 4 to modulator 1118, and frequency control signal n to modulator1120. In binary frequency shift keying (BFSK), the respective frequencycontrol signals provide data corresponding to a binary "one" or "zero"which causes the respective modulators to modulate a first or secondfrequency onto the carrier signal.

A summer 1122 combines the modulated carrier frequencies to produce anoutput signal.

3. M'ary Frequency Shift Keying Modulation

A modulation scheme related to binary frequency shift keying is M'aryfrequency shift keying. M'ary frequency shift keying modulates three ormore different frequencies onto the respective carrier signals. Inquaternary frequency shift keying, for example, two bits of informationmay be instantaneously conveyed on a single carrier frequency.Similarly, 8'ary frequency shift keying may instantaneously convey threebits of information per carrier frequency.

Referring again to FIG. 11, M'ary frequency shift keying may beimplemented by providing modulators 1112, 1114, 1116, 1118, and 1120with the capability to modulate M different frequencies onto the carriersignal. Accordingly, the various frequency control signals must providedata indicating which of the M frequencies is to be modulated onto thecarrier signal. For example, in quaternary frequency shift keying, thefrequency control signals must each include two bits of information toindicate which of the four different frequencies are to be modulatedonto the carrier frequency.

The summer 1122 combines the modulated carrier frequencies to produce anoutput signal.

4. Quadrature Amplitude Multi-Carrier Modulation

Yet another alternative modulation technique for a multi-carriertransmission format is shown in FIG. 12. A quadrature modulator 1200includes a first quadrature carrier generator 1202, a second quadraturecarrier generator 1204, a third quadrature carrier generator 1206, and afourth quadrature carrier generator 1208. As is well known, quadraturemodulators: in general each produce an in-phase carrier signal and aquadrature carrier signal that is +/-90° out of phase with reference tothe in-phase signal. Of course, any number of quadrature carriergenerators could be envisioned, depending upon data transfer andthroughput needs. FIG. 12 shows four quadrature carrier generationswhich effectively correspond to eight unique modulator signals.Therefore, quadrature amplitude multi-carrier modulation may preferablyreduce the width of the frequency band necessary to achieve a desireddata transfer rate.

Each quadrature carrier generator 1202, 1204, 1206, and 1208 receives acontrol signal from a control logic system (not shown) which providesthe data to be modulated onto the quadrature carrier signals. In asimple implementation, the quadrature carrier generators may amplitudemodulate the in-phase and quadrature phase output signals to convey twobits of information. The in-phase and quadrature signals output fromeach quadrature carrier generators 1202, 1204, 1206, and 1208 areprovided to a summer 1210 which combines the signals to produce anoutput signal.

5. Permutation Frequency Shift Keying (PFSK)

PFSK may be implemented through control logic systems similar to thatused in a MOOK or an M'ary FSK modulation scheme. In PFSK, every baudhas a fixed number of carrier signals present, preferably any 4 of thepossible 8. In a PFSK arrangement, a constant average transmitter poweris advantageously delivered and the receiver only need decide which 4carrier frequencies contain the most energy. In the case of MOOK, thereceiver must attempt to determine on a subchannel-by-subchannel basisthe presence or absence of a signal. This aspect of PFSK may simplifymobile receiver design.

Compared to a binary or M'ary FSK modulation schemes, a higher number ofbits may be delivered per baud with PFSK. For example, PFSK may generatesignals that independent FSK subchannels could never generate, such asall four carriers being the four highest frequencies, and therefore itcan be seen that PFSK may advantageously increase information transferrates.

D. The Base Transmitter

Each base transmitter unit, such as base transmitter 612 or 614 shown inFIG. 6, receives transmitter control data and message data transmittedfrom the satellite 606. FIG. 13 shows a first preferred embodiment of abase transmitter 1300 in accordance with the present invention. The basetransmitter 1300 receives data from the satellite downlink connected todata input 1302 which provides this data to a control logic system 1304to control the operation of the base transmitter unit. The control logic1304 provides a control signal to a plurality of modulators 1306, 1308,1310, 1312, and 1314. Modulator 1306 produces a carrier signal F1,modulator 1308 produces a carrier signal F2, modulator 1310 produces acarrier signal F3, modulator 1312 produces a carrier signal F4, andmodulator 1314 produces a carrier signal Fn.

For example, the control logic may generate appropriate control signalsto modulate the carrier signals in a MOOK, BFSK, M'ary FSK, PFSK, orquadrature amplitude modulation scheme, as previously discussed. Eachmodulator then provides the modulated output signal to a combiner 1316which combines each of the several modulated carrier frequencies into asingle output signal.

The single signal is then applied to a power amplifier 1318 to amplifythis signal to an appropriate level. The power amplifier 1318 may, forexample, produce a nominal output signal of 350 watts to antenna 1320.In this embodiment, power amplifier 1318 preferably has extremely linearcharacteristics to prevent formation of intermodulation products, and toinsure that these intermodulation products do not cause signals to begenerated at undesirable frequencies. Antenna 1320 broadcasts thedesired signal from power amplifier 1318.

FIG. 14 shows a second preferred embodiment of a base transmitter unit.The second embodiment comprises a base transmitter 1400 which includes asatellite downlink connected to data input 1402, control logic 1404, andseveral modulators 1406, 1408, 1410, 1412, and 1414. Each modulatorreceives an appropriate control signal from the control logic 1404, aspreviously discussed with respect to base transmitter 1300.

The output from each of modulators 1406, 1408, 1410, 1412, and 1414 inbase transmitter 1400 is provided to respective power amplifiers 1416,1418, 1420, 1422, and 1424 to provide an appropriate power output levelfor transmission, such as 350 watts aggregate.

The output from each of power amplifiers 1416, 1418, 1420, 1422, and1424 is provided to combiner 1426 to combine the modulated carriersignals into a single output signal which is provided to antenna 1428for broadcast.

E. The Mobile Unit

The mobile unit may be a small, portable mobile transceiver, such aspictorially represented in FIG. 16. Referring now to FIG. 15, the mobiletransceiver 1500 shown therein includes a receiver section for receivingsignals from the base transmitters of the system, and a transmittersection for transmitting replies, or other messages, to the basereceivers of the system.

In particular, the mobile transceiver 1500 includes an antenna 1502which is connected to a transmit/receive switch 1504 to switch theantenna between the transmit and receive sections of the mobiletransceiver 1500. A receiver 1506 is provided to receive the messagesfrom the base transmitter. Of course, the receiver must be appropriatelydesigned to receive the multi-carrier signals from the base transmittersand must be appropriately designed to demodulate the particularmodulation scheme utilized. For example, appropriate analog filters andappropriate demodulators could be used. In the preferred embodiment, thereceiver performs a transform, such as a fast fourier transform, on thereceived signal to separate the data from the various carriers in themulti-carrier modulation format.

The receiver 1506 is connected to a display and storage logic section1508 to process the received signal. An annunciator 1510 to alert theuser that a message has been received is connected to and controlled bythe display and storage logic 1508. The annunciator 1510 may commonlyinclude a sound producing device such as a beeper, or a vibrator, or aflashing light.

A set of display controls 1512 to control the display of the mobiletransceiver 1500 is connected to the display and storage logic 1508. Adisplay 1514, preferably an LCD display, is also connected to thedisplay and storage logic 1508 to display messages and various otherinformation to the user.

Display and storage logic 1508 is connected to transmit logic 1518 viaconnection 1526. Display and storage logic 1508 may generate anautonomous acknowledge signal which causes the transmitter 1520 tobroadcast an appropriately modulated RF signal. As previously discussed,it is desirable for the mobile transceiver to transmit an acknowledgesignal if the message was properly received by the mobile unit, oralternatively to transmit a negative acknowledge signal if the messagewas only partially received. The negative acknowledge signal indicatesthat the network operations center should rebroadcast the message to themobile unit.

Preferably, the rebroadcast of the message to the mobile unit shouldoccur with an appropriate error correcting code which may be decoded bythe mobile unit to insure complete and accurate reception of themessage. Of course, error correcting codes should be used only whennecessary because their use slows data transfer and increases thecomplexity of the mobile unit. Other types of autonomous replies mayalso be useful, for example, to indicate to the network operationscenter that the user has not viewed the message even though the mobileunit properly received it, such as when the mobile transceiver isunattended by the user.

A set of input switches 1516 is provided to allow the user to input areply to a received message, or to otherwise generate a message to betransmitted by the mobile transceiver. The input switches are connectedto transmit logic 1518 which decodes the signal from the input switches1516 to generate an output signal to the transmitter 1520. Thetransmitter 1520 generates an appropriately modulated RF signal to bebroadcast by antenna 1502.

The mobile transceiver 1500 also preferably includes a noise detector1522. The noise detector 1522 provides an output signal upon sensingthrough antenna 1502 a threshold level signal. The noise detector 1522provides an output signal to disable the transmitter 1520 via connection1524, and to thereby prevent unwanted transmission by the mobile unit.

Noise detector 1522 preferably is set to detect electromagnetic signalswhich are generated externally to the communication system and which areindicative of a condition when transmissions by the mobile unit areundesirable. For example, the noise detector 1522 could be designed toserve a threshold level of noise at 400 Hz. When the user enters acommercial aircraft, which commonly uses 400 hertz power supply, thereceipt of this noise by the noise detector 1522 would then disable thetransmit capability of the mobile transceiver 1500 during operation ofthe aircraft to prevent any unnecessary or unwanted interference withthe operations of the aircraft by autonomous or intentionaltransmissions by the mobile transceiver 1500.

The display and storage logic 1508 of the mobile transceiver 1500further preferably includes a timing circuit (not shown) which may beused to turn the receiver section 1506 on or off, as desired. The timingcircuit (not shown) advantageously allows the mobile transceiver to"power down" during periods of time when messages are not anticipated tobe transmitted. For example, in a preferred communication protocol, thereceiver could simply power up at the beginning of each cycle to receivedata to determine if a message will be transmitted to that mobiletransceiver during that cycle or when information concerning messageavailability will be transmitted. If the mobile transceiver is toreceive a message, the timing circuit could power up at the appropriatetime to receive the message, and then power down after receipt. Thetiming circuit, therefore, advantageously prolongs the battery life ofthe mobile transceiver 1500. Of course, it should be understood that thetiming circuit could control the other elements of the mobiletransceiver, such as the display 1514, and the transmit logic 1518.

In an alternate implementation, the receiver 1506 may adaptively changeits demodulation techniques to accommodate various formats. For example,each zone may advantageously use a different modulation format dependingon message traffic levels, and other considerations. In particular, thereceiver may receive a signal indicating the modulation scheme utilizedin a given zone via a modulation format message contained in an overheadportion of the data stream. The demodulation of FSK, M'ary FSK, PFSK,and MOOK formats all begin with the determination of the energy levelsdetected at each of the carrier frequencies, and thus require identicalprocessing of the received RF energy. The logic (not shown) in thereceiver interprets the meaning of these measured energy levels basedupon the modulation scheme selected as indicated by the receivedmodulation format message. In this manner simpler and more economicaltransmitters, with a decreased capacity for information transfer, can beused in zones that have decreased traffic loads and more expensive,high-throughput transmitters can be used only in those areas where theyare needed.

A pictorial representation of the mobile transceiver is shown in FIG.16. The mobile transceiver 1600 shown therein includes a case 1602, apair of display control buttons 1604, a display 1605, a set of six replybuttons 1608, 1610, 1612, 1614, 1616, and 1618. As indicated previously,display 1606 is preferably an LCD display and a set of display controlbuttons 1604 may be used to scroll text up or down on the display 1606.The message "will you be home for dinner?" is shown on display 1606.

The set of six reply buttons 1608, 1610, 1612, 1614, 1616, and 1618provide a flexible system for user generated replies to receivedmessages. The display and storage logic 1508 provides informationimmediately above each button indicating a possible reply message by theuser. In the simple example shown in FIG. 16, the user may reply "yes,""no," or "?" to the message 1620 displayed on the screen 1606. Thetransmit logic 1518 generates an appropriate signal based upon whichbutton the user presses. In this simple scenario, buttons 1614, 1616,and 1618 are unused.

In alternate applications, up to six possible reply messages; may beshown on the screen 1606. Of course, other particularized applicationsmay be envisioned for the reply feature of the mobile transceiver 1500.For example, if the user is a stockbroker, the display 1606 coulddisplay the terms "buy," "sell," or "hold" above the appropriatebuttons. A variety of other applications may be envisioned.

With the six button reply option provided by mobile transceiver 1500, athree bit message may be transmitted by the mobile transceiver to thebase receivers. The two remaining states of the three bit message may beused by the transmit logic 1518 for the autonomous acknowledgment signalwhich indicates that the message has been properly received, and for theautonomous negative acknowledgment signal which indicates that themessage has not been completely or properly received.

Of course, the mobile transceiver 1500 shown in FIG. 16 could beconfigured differently to provide more or less reply buttons, differentdisplay control buttons, and different display formats as desired orneeded by the user.

Further, the mobile transceiver 1500 could additionally include a dataoutput port (not shown) for connection to other electronic devices ofthe user. For example, the mobile transceiver could be connected throughan output port to a laptop or palmtop PC, or could be incorporatedtherein. The PC could display the message on its screen, therebyobviating the need for the display 1606, and the keyboard could be usedto generate any appropriate reply messages from the user, therebyobviating need for the reply buttons and allowing free form messages tobe sent by the mobile transceiver. A user selected reply would betransferred to the mobile transceiver 1500 from the PC for transmissionto the base receiver.

Alternatively, the mobile transceiver could be connected to a voice datareplay device, such as a speaker, thereby allowing the user to receivemessages from a voice mailbox, for example. Of course, a voice datageneration device, such as a microphone, could be connected to themobile transceiver 1500 to allow the user to reply to the voice mailmessage he has received or to initiate voice data communication from themobile transceiver to the base receivers. Similarly, facsimiletransmissions could be supported.

An alternate embodiment of the mobile unit includes only receivecapabilities, but does not include any transmit capabilities. FIG. 17shows a mobile receiver 1700. The various components of the mobilereceiver generally correspond in functionality to the similar elementsshown in FIG. 15. Of course, the mobile receiver 1700 cannot generatereplies, which includes user initiated replies, an autonomousacknowledgment signals or negative acknowledgment signals, because ofthe lack of transmit capability. Also, the location of this alternateembodiment cannot be tracked by the network control center because ofthe lack of transmit capability. Generally, because of these reasons,the mobile receiver 1700 embodiment of the mobile unit is lesspreferable than the mobile transceiver embodiment 1500. Further, itshould be appreciated that the mobile transceiver embodiment may includecircuitry for generating various autonomous responses withoutinteraction by the user.

F. The Base Receiver

The base receivers of the present system receive the low power outputsignal from the mobile transceiver unit. As is shown in FIG. 6, mobilereceivers are dispersed throughout the geographic service area. Basereceivers need not be associated with zonal boundaries per se, but willalways be located to service at least one zone, of course. A few basereceivers may exist in the overlap region between zones.

During transmission of the return signal by the mobile transceiver unit,it is possible that several base receivers could receive this returnsignal. In this instance, the network operations center 600 preferablyselects the data from the base receiver with the highest received signalstrength (i.e. the signal with the lowest probability of errors) tomaximize the likelihood of receiving accurate data. The signal strengthapproach is preferred and can be satisfactorily implemented if the basereceiver locations are carefully selected to insure adequate signalstrength reception from the mobile transceiver units and to minimize theoverlap between base receiver coverage areas. Alternately, the networkoperations center 600 could use "voting" techniques by comparing eachdata set from the several base receivers to arrive at the most likelyreturn signal data using conventional voting receiver technology.

FIG. 18(A) shows a first embodiment of an analog base receiver. Analogreceiver 1802 is connected to an antenna 1800. The analog receiver 1802simply receives the signal from the antenna 1800 and removes themodulated waveform from the carrier frequency and outputs this waveformin analog format to a regional demodulator 1804 via data path 1806. Datapath 1806 is preferably a 4 KHz analog telephone channel.

The regional demodulator 1804 receives signals from several analogreceivers included in several base receivers. Preferably, the regionaldemodulator 1804 is located in the regional station, such as regionalstation 650 shown in FIG. 6. The demodulated signal from the regionaldemodulator 1804 is then transferred to the regional processingcircuitry 1808, and then onto the network operations center 600.

The analog receiver 1802 could generate identification data to betransmitted with each received message so the network operations center600 can determine the source of each message received. Alternatively,and preferably, dedicated communication paths are used for each basereceiver and therefore, the source of the message can be inferred fromthe communication path that is activated.

FIG. 18(B) shows a digital base receiver embodiment which includes anantenna 1800 attached to an analog receiver 1802. As in the previouslydiscussed embodiment, the analog receiver 1802 removes the modulatedwaveform from the carrier signal transmitted by the mobile transceiverunit. The analog receiver 1802 outputs the modulated waveform to ademodulator 1810 included in the base receiver. The demodulator 1810produces a digital output signal corresponding to the data streamtransmitted by the mobile transceiver unit. The demodulator 1810provides the digital output signal to the regional processing circuitry1808 in the regional station via data path 1812. Data path 1812 may beany conventional data path which can satisfactorily convey the digitaldata from the demodulator 1810 to the regional processing center 1808.The regional processing circuitry 1808 then passes the data to thenetwork operations center 600.

FIG. 19 shows a digital base receiver including error correction andstore and forward features. An antenna 1900 is connected to an analogreceiver 1802 which is connected to a demodulator 1810, as previouslydescribed with reference to FIG. 18(B). The demodulated digital signalis output from demodulator 1810 to error correction circuitry 1906 whichmay perform error correction algorithms to insure the integrity of thereturn signal received from the mobile transceiver unit. Of course, theerror correction circuitry should decode and correct data which havebeen compatibly encoded by the mobile transceiver.

The error corrected data output from the error correction circuitry 1906is provided to a store and forward circuit 1908. The store and forwardcircuit 1908 stores the received data to allow it to be transmittedlater at a convenient time and at a convenient data transmission rate.

For example, in the present system it is likely that the return signaltraffic received by the base receiver will occur in short bursts at arelatively high data transfer rate. However, it is also likely that theaverage data transfer rate from the base receivers is substantiallylower than the instantaneous data transfer rate during traffic bursts.The store and forward circuit 1908 may preferably act as a buffer toallow the return signal data to be communicated from the store andforward circuit 1908 to the regional processing circuitry 1808 at alower (and less expensive) data transfer rate. Store and forward circuit1908 is, therefore, preferably connected to regional processingcircuitry 1808 via data path 1910 which may include a low cost telephoneline.

G. The Network Operations Center 1. Overview

The network operations center 600 is shown in schematic form in FIG. 20.The network operations center 600 includes a base receiver input system2000 which receives data from the various regional stations throughoutthe system (e.g., regional stations 644 and 650) via various data paths,such as data paths 656 and 658 as shown in FIG. 6. The data received bythe base receiver input system 2000 includes reply data from users withvarious control data. Base receiver input system 2000 may includeappropriate conventional signal processing equipment. Control data mayinclude data identifying the base receiver (i.e. location of the mobileunit) which received the associated reply. Preferably, the base receiverinput section 2000 receives data from the regional stations via phonelines. However, other appropriate data paths may be considered.

The base receiver input system 2000 then provides the received data to acentral computer 2002. The central computer 2002 may also receive inputfrom a user input system 2004. For example, the user input system 2004may receive data from users via phone lines who may access and interactwith the central computer via voice, DTMF, or modem transmission and mayinclude appropriate conventional signal processing equipment. A user mayinteract with the central computer 2002 to modify his service, toinitiate or receive messages, or to perform other desirable functions.

Generally, the central computer 2002 processes the data received fromthe base receiver input system 2000 and from the user input system 2004to perform various operations on the data, to update various databaseentries for use by the central computer 2002, and to generate data fortransmission to a satellite uplink output system 2006.

It should be understood that, although FIG. 20 shows the centralcomputer as existing at a single location in the network operationscenter 600, a distributed computing system may be used to perform thenecessary functionality of the central computer 2002. Presently,however, a single location for the central computer 2002 is preferred.

Satellite uplink output system 2006 receives data from the centralcomputer 2002 and provides it to satellite 606, shown in FIG. 6, fortransmission to base transmitters within the system (e.g., basetransmitters 612 and 614 in FIG. 6).

The central computer 2002 is also connected to a database system 2008which stores various data such as message data, user status data, systemstatus data, and message status data, for example, for use by thecentral computer 2002 in processing.

Also, a control access 2010 is provided to allow systems engineers orprogrammers to access the central computer 2002 to observe and modifyits operations and system performance.

2. Database Structure

The database 2008 of the network operations center includes severaldatabase structures necessary for the operation of the system. While apreferred partitioning of these databases is described below, it shouldbe understood that other partitionings could be considered, such asmoving the various "user traffic" fields from the traffic statisticsdatabase to the user database.

a. The User Database

For example, the user database structure shown in FIG. 21 includes arecord for each user of the system who possesses a mobile unit. Therecord for user 1 2100 includes various fields, such as an ID numberfield 2102 which indicates a unique number associated with thatparticular user. The transmit capability field 2106 indicates whetherthe mobile unit assigned to the user has the capability to transmit. Thelast location field 2104 includes data which indicates the last knownlocation of the user. The last location field may be updated when thecentral computer recognizes that a new base receiver has received areturn signal from the mobile unit, thereby indicating the mobile unithas moved since the last return signal. Of course, if the mobile unitonly includes a mobile receiver without transmit capability, the lastlocation field 2104 cannot be updated and the mobile unit may be given adefault location.

The service area field 2108 includes data corresponding to the area inwhich the user has subscribed to. For example, if a user desires servicein geographic areas less than the total system service area, the centralcomputer could use the data in the service area field 2108 to cause onlyselected base transmitters to attempt to transmit messages to a mobileunit.

The button format field 2110 includes data indicating the format ofreply buttons the user may access on the mobile transceiver. Of course,for mobile units with only receive capabilities, the button format fieldwill not be used.

The message field 2112 includes data representing one or more messageswhich are intended for the user. A receive flag is set when the centralcomputer has received data indicating that the message has been receivedby the mobile unit via an acknowledgment signal. If the mobile unit doesnot have transmit capability, the receive flag is set upon transmissionof the message by the appropriate base transmitters. The user databasestructure may include other fields for each user of the communicationsystem of the present invention as needed to provide various desiredservices.

b. The Receiver Database

Database 2008 of FIG. 20 includes a receiver database (not shown) whichincludes an entry with several associated fields for each base receiverin the system. A first field for each base receiver preferably includesthe total number of mobile units which have last communicated with thisreceiver. A second field for each base receiver preferably includes alist of base transmitters which may cover all or a portion of thereceiver coverage area of that base receiver.

c. Traffic Statistics Database

Database 2008 of FIG. 20 should also include preferably a trafficstatistics database as shown in FIG. 22 which includes various fieldscontaining statistics calculated by the central computer 2002 concerningtraffic patterns for the system. For example, the traffic database 2200preferably includes a user field 2202 for data indicating a user of thenetwork. Several fields are preferably associated with the user field2202. Field. 2204 includes data representing the number of probe signalssent by the network to locate the mobile unit associated with the userfield 2202. Field 2206 includes data representing the number ofregistration signals received by the network from the mobile unitassociated with the user field 2202. Field 2208 includes datarepresenting the number of messages from the network that have beensuccessfully delivered to the mobile unit associated with the user field2202. Field 2210 may be used for other traffic related data, such asdata indicating the average traffic per cycle, and data indicating atime average (i.e. for the last hour) traffic amount.

Further, the traffic database 2200 could include fields (not shown) fordata concerning overall system performance and, in particular, each zonein the network. Such area specific traffic data may be useful inoptimizing system performance by allowing intelligent redefinition ofzonal boundaries.

d. The Service Queue

Database 2008 of FIG. 20 also includes a service queue 2300 as shown inFIG. 20. The service queue 2300 includes a current messages queue and aprobe list queue. The current messages queue includes a system wide listof messages to be delivered by the system. The current messages queueincludes, for example, a series of ID number fields 2302, 2304, and 2306with associated data location fields 2308, 2310, and 2312, respectively.The data location fields 2308, 2310, and 2312 include pointers to theappropriate fields in the user database structure shown in FIG. 21. TheID number fields 2302, 2304, and 2306 include data indicating the IDnumber of the user to which the message is to be delivered.

In operation, the central computer retrieves the ID number 2302 and datalocation 2308 from the top of the current messages queue and retrievesthe appropriate data from the user database 2100 to process and transmita message to the user.

The probe list queue includes a ID number fields 2314, 2316, and 2318and data location fields 2320, 2322, and 2324 similar in form to thosein the current messages queue. The probe list queue contains a list ofusers which the system has previously attempted unsuccessfully todeliver a message to. In other words, the users listed in the probe listare considered to be "lost" by the system. The central computer 2002then initiates a probe routine for the ID number 2314 and data location2320 located at the top of the probe list.

After successful execution of the probe routine, the last location field2304 in the user database structure 2100 will have been updated toprovide an accurate last location of the user from the base receiverthat received the mobile unit's acknowledgment to the probe signal.After the last location field 2304 has been updated, the message canthen be replaced in the current messages queue for delivery to the uservia the appropriate base transmitters located near the mobile unit.

Preferably, the network operations center gives priority to the deliveryof all messages in the current message queue, and then sends probesignals to the users listed in the probe list queue after delivery hasbeen attempted for all messages in the current message queue. If themessage volume in the current message queue remains high for an extendedperiod of time, the network operations center preferably begins toperiodically send probe signals to the users listed in the Probe List,even though undelivered messages remain in the current messages queue.For example, in this instance of persistent filled current messagesqueue, the network operation center preferably transmits three probesignals in every cycle transmitted.

e. Base Transmitter Assignment List

The database 2008 of the network operations center also includes a basetransmitter database 2400 as shown in FIG. 24. The base transmitterdatabase 2400 includes a zonal assignment field 2404 for datarepresenting a zone assignment associated with a base transmitter field2402 in the system. Also, a field 2406 for data representing the basereceivers in the transmitter coverage area, and a field 2408 for otherdata associated with a base transmitter, are associated with basetransmitter field 2402. As can be seen in FIG. 24, each base transmitterin the network has a base transmitter field and associated fields asdescribed above.

In normal operating conditions of the system with low amounts of messagetraffic being transmitted, each base transmitter will remain assigned toits particular zone. However, the systems and methods of the presentinvention provide for dynamically changing the zonal assignments ofvarious base transmitters to improve information throughput. Thesedynamic zone allocation concepts dynamically reassign base transmittersto new zones generally based upon the volume of messages transmittedduring the systemwide time interval, and more particularly based uponthe localized volume of messages to mobile units. In general, dynamiczone allocation may be used to deliver messages to mobile units inoverlap areas (i.e. "zonal dithering"), or to balance the volume ofmessage traffic between zones.

FIG. 25 is useful to explain these concepts. Various base transmitters,each designated as an "X," are dispersed throughout a region of spaceshown in FIG. 25. Also, various base receivers are dispersed throughoutthis region of space 2500, each being designated by an "R." The normalzonal boundary for zone 1 in FIG. 25 is shown by solid line 2502. Anormal boundary for zone 2 is represented by solid line 2504 duringnormal load traffic operation conditions. As can be seen, basetransmitters 2506, 2508, and 2510 are located near the zonal boundary ofzone 2, and base transmitters 2512, 2514, and 2516 are located near theboundary of zone 1. Base receivers 2518 and 2520 are located in anoverlap area 2521 between zones 1 and 2. As previously discussed, mobileunits located in this overlap area 2521 near base receivers 2518 and2520 must be communicated with during the systemwide time intervalbecause of the interference created during the zonal time interval byadjacent base transmitters.

During normal, low to moderate volume system operations, the zonaloverlap area 2521, i.e., interference area, near base receivers 2518 and2520 will preferably have a small number of mobile units locatedtherein. Therefore, communication with these mobile units will notsignificantly consume system resources by occasionally communicatingwith them during the systemwide time interval.

However, if the traffic volume from the overlap area 2521 near basereceivers 2518 and 2520 increases, such as because additional mobileunits enter this overlap area 2521, the handling of this traffic in thesystemwide time interval can significantly consume system resources. Forexample, communication with a large number of mobile units during thesystemwide time interval may significantly delay delivery of messages tounits in this and other regions.

In this instance, the zonal boundaries are changed to remove this hightraffic region from a zonal overlap area. For example, system efficiencyis restored if the zone 1 boundary were moved to dashed line 2522 andthe zone 2 boundary were moved to dashed line 2524.

The central computer 2002 may dynamically accomplish this zonalredefinition by assigning one or more base transmitters to a new zone toreduce systemwide time interval messages. In the present example shownin FIG. 25, the central computer updates the base transmitter zonalassignment list to reassign base transmitters 2512, 2514, and 2516 tozone 2 while removing these base transmitters from zone 1. In view ofthis zonal redefinition, the new zone 1 boundary is shown by dashed line2522, and the new zone 2 boundary is shown by dashed line 2524. The hightraffic region near base receivers 2518 and 2520 is now squarely withinzone 2 and messages to these units may be efficiently delivered duringsubsequent zonal time interval(s).

In accordance with the invention, a preferred method 2600 foraccomplishing zonal redefinition is shown in FIG. 26. In accordance withthe method, step 2602 provides for transmitting substantiallysimultaneously a first information signal and a second informationsignal, the first information signal being transmitted in simulcast by afirst set of base transmitters assigned to a first zone, and the secondinformation signal being transmitted in simulcast by a second set ofbase transmitters assigned to a second zone. For example, as shown inFIG. 25, the base transmitters in zone 1 defined by boundary line 2502could be the first set of base transmitters, and the base transmitterslocated in zone 2 defined by boundary line 2504 could be the second setof base transmitters.

Step 2604 of the method provides for dynamically reassigning one or moreof the base transmitters in the first set of base transmitters assignedto the first zone to the second set of base transmitters assigned to thesecond zone, thereby creating an updated first set of base transmittersand an updated second set of base transmitters. For example, basetransmitters 2512, 2514, and 2516 could be reassigned from zone 1 tozone 2. As shown in FIG. 25, new zonal boundaries would be defined bydashed lines 2512 for zone 1 and 2524 for zone 2.

Step 2606 provides transmitting substantially simultaneously a thirdinformation signal and a fourth information signal, the thirdinformation signal being transmitted in simulcast by the updated firstset of base transmitters and the fourth information signal beingtransmitted in simulcast by the updated second set of base transmitters.For example, as shown in FIG. 25, the base transmitters assigned to zone1 defined by dashed line 2522 (i.e. not including base transmitters2512, 2514, and 2516) could transmit during a subsequent communicationcycle a third information signal, and base transmitters in zone 2defined by dashed line 2524 (i.e. including base transmitters 2512,2514, and 2516) could transmit a fourth information signal during thatsame subsequent communication cycle.

Further, it is desirable that during the redefinition of the zonalboundaries, it is insured that the new overlap area 2525 near basereceiver 2526 and between dashed lines 2522 and 2524 is an area that isnot likely to produce, or is not currently producing a high volume ofmessage traffic. Generally, zonal boundaries should be preferablyredefined to maximize information throughput by minimizing the data thatmust be transferred during the systemwide time interval. A networkmanager could review the overall traffic patterns and tendencies todetermine an optimum redefinition of zonal boundaries. Of course, thecentral computer 2002 could also implement an algorithm accessing thetraffic statistics database 2200 to determine optimal zonal boundaryredefinition.

In a preferred embodiment in the instance where an entire region issaturated with mobile units, such as a large metropolitan arearepetitive reassignments of base transmitters may be used to reducemessage traffics during the systemwide time interval. There may exist noappropriate overlap area, such as overlap area 2525, with a low trafficlevel to facilitate a long term reassignment of base transmitters withthe resulting redefinition of zonal boundaries. In this case, thepreferred embodiment alternates between a first and second set of zonalboundaries over each communication cycle and does not attempt to delivermessages during the systemwide time interval.

For example, in FIG. 25 this preferred embodiment would utilize thezonal boundaries defined by lines 2502 and 2504 during a first zonaltime interval and would not attempt to deliver messages to mobile unitsin overlap area 2521. In a subsequent cycle, this preferred embodimentredefines the zonal boundaries to dashed lines 2522 and 2524 anddelivers messages to the mobile units in previous overlap area 2521during the zonal time interval using zone 2 base transmitters. Duringthis cycle, the network would not attempt to deliver messages to mobileunits in overlap, area 2525. In yet a later cycle, this preferredembodiment would switch back to zonal boundaries 2502 and 2504 whichwould allow message delivery to mobile units in the now previous overlaparea 2525 during the zonal time interval using zone 1 base transmitters.As can be seen, alternating between a first and second set of zonalboundaries advantageously reduces the need for communication during thesystemwide time interval, but slows message delivery somewhat by onlyallowing communication to mobile units in overlap areas during zonaltime intervals on alternating communication cycles.

H. The Preferred System Communication Protocol

The system communication protocol is preferably a time division protocolorganized within repetitive communication cycles of preferably 30seconds in duration.

The blocks of data transmitted by the network are preferably formed by abit interleaving process to prevent loss of data during bursts ofinterference. Bit interleaving may be envisioned as stacking two or moreblocks of data (which read from left to right), and then transmitting abit stream in a column-by-column, top-to-bottom sequence. As can beseen, a burst of interference will likely only cause the loss of a fewbits per word at most, which can be corrected by error correctiontechniques, rather than the loss of entire words. Of course, the mobileunit must appropriately deinterleave the data prior to processing.

FIG. 27 generally illustrates a variety of preferred time intervalswhich may variously be used for communication between the system andvarious sets and subsets of mobile units. An adaptable schedule forthese time intervals is preferably generated, and may be revisedaccording to system demands. The scheduling of the time intervalsadvantageously allows a mobile unit to "power down" during inactive timeperiods when the mobile unit will not transmit or receive any messages,thereby conserving battery power. Similarly, messages or information fordelivery to a subset of the total number of mobile units will preferablybe transmitted during time intervals which minimize the delivery ofthose messages or information to unintended mobile units not included inthe subset to further conserve battery power.

A preferred cycle protocol 2700 is shown in FIG. 27(A). The cycleprotocol 2700 includes a cycle header time interval 2702, a systemwideforward (FWD) batch time interval 2704, a systemwide response timeinterval 2706, a zonal forward (FWD) batch time interval 2708, a zonalreverse time interval 2710, and a reverse contention time interval 2712.Other arrangements, such as moving the systemwide reverse interval nextto the zonal reverse interval may be considered if transmitter turn ontime is significant.

The cycle protocol generally schedules time slots for systemwide andzonal forward channel information transfer from the network to themobile units and for systemwide and zonal reverse channel informationtransfer from the mobile transceiver units to the network. Briefly, thecycle header 2702 field includes overhead or "housekeeping" information,the systemwide forward batch field 2704 and the zonal forward batchfield 2708 provide forward communication capability through the basetransmitters to the mobile units in a systemwide time interval and azonal time interval, respectively. The systemwide response field 2706and zonal reverse field 2710 provide a return signal period for themobile transceivers to respond to messages generated during thesystemwide and zonal forward batch periods 2504 and 2508, respectively.Finally, the reverse contention 2712 field allows the mobile transceiverto initiate access to the network.

Each of the fields shown, except the cycle header 2702 field, ispreferably variable in duration, and may be changed by the centralcomputer 2002, depending on message traffic requirements. The beginningof the cycle is synchronized by the central computer to a time standardand preferably coincides with the start of minute or half minuteintervals. Each mobile unit preferably includes timing circuitry, aspreviously described, which allows for the mobile unit to power up atthe beginning of each cycle to receive communication.

For each cycle, the central computer 2002 calculates the amount of timerequired for each field to maximize information throughput by thenetwork. For example, for the cycle protocol 2700 shown in FIG. 27(A),the central computer will calculate the amount of time necessary for thesystemwide forward batch field 2704, the systemwide response interval2706, the zonal forward interval 2708, the zonal reverse interval 2710,and the reverse contention interval 2712. The cycle header 2702 willpreferably include timing offset data which will indicate the timingoffset from the cycle header until the beginning of the systemwideresponse interval 2706, the beginning of the zonal forward interval2708, the beginning of the zonal reverse interval 2710, and thebeginning of the reverse contention interval 2712.

The cycle header 2702 starts preferably with an 8 digit long preamble(not shown) for digit synchronization purposes. The preamble allows forthe mobile unit to synchronize its timing circuitry with the network.For example, the timing circuitry of the mobile unit could become offsetfrom the network due to commonly caused inaccuracies. The preamble isfollowed by a "start of header" string of four digits and all timingoffsets within the cycle are calculated as a number of predefinedintervals beginning from the start of the last header digit. The startof header string is followed by an 8 digit string grouped into twowords, each of which is protected against errors by encoding it using aforward error correcting code, preferably a Bose, Chaudhuri, andHocquenghem (BCH) code or a Reed Solomon code. These error correctingcodes add additional digits to the information digits in a code word,where the additional digits are a specific function of the informationdigits, so that if certain common error events occur, a decoding stepinvolving all of the transmitted digits, both information andadditional, can recover the original information digits. The first codeword will contain a count of the current cycles executed for that day.The second code word will contain the necessary timing offsets for thebeginning of the time intervals in the cycle protocol 2700. Furtherinformation regarding error correcting codes may be found in Gallagher,"Information Theory and Reliable Communication," Wiley 1968, which ishereby incorporated by reference.

The systemwide forward batch 2704 field generally includes a zonalheader time interval including overhead information and a series of 64batches. Also, the zonal forward interval 2710 similarly includes azonal header time interval with overhead information and a series of 64batches. Each batch is a string of data containing informationspecifically directed to a single group of mobile units. Each batchpreferably contains information directed to a certain class of mobileunits with the classes divided by the types of service provided. Forexample, a first batch could be directed to all mobile transceiverunits, and a second batch could be directed to all mobile receiverunits. Further, each batch may contain several messages, each intendedfor different mobile units within the particular class of unit to whichthat batch is directed. Generally, FIG. 27(B) shows the forward batchinterval protocol 2750 preferred for both the systemwide forwardinterval 2704 and the zonal forward interval 2708.

The systemwide forward interval 2704 is preferably used only for sendinga probe signal to a mobile transceiver unit which does not respond tozonal messages (i.e. a "lost" unit). However, when necessary, thesystemwide forward interval 2704 may be used to deliver messages tomobile units located in overlap areas. The ID number, or address, of thelost mobile unit is preferably followed by data indicating a timingoffset which is a time delay amount until the beginning of the time slotdesignated for the return signal of that mobile unit. An alternativeimplementation, which may be useful for mobile units that have notresponded for a period of time, could have mobile units that havereceived a probe signal respond during the reverse contention interval.

After the end of the broadcast on the systemwide forward batch timeinterval 2704, all network base transmitters shut down until thebeginning of the zonal forward batch time interval 2708.

The forward batch interval protocol 2750 includes a forward channelheader interval 2714 which includes data to allow the timing circuitryof the mobile units to synchronize themselves with the incoming datastream. The forward channel header 2714 also preferably includes dataindicating a timing offset scheduling a reverse channel time intervalfor each batch, as may be required. Of course, the forward channelheader 2714 for the systemwide forward interval 2704 would indicate atiming offset for reverse channel transmission during the systemwideresponse interval 2706, and the forward channel header 2714 for thezonal forward interval 2708 would indicate a timing offset for reversechannel transmission during the zonal reverse interval 2710.

The forward channel header 2714 further includes a data stream to themobile unit listing which of the 64 batches will follow and the timingoffsets indicating when those batches will be transmitted. Again, thisfeature advantageously allows the mobile unit to "power down" during thesystemwide and zonal forward intervals 2704 and 2708 until theappropriate time for receiving its batch information, thereby conservingthe battery power of the mobile unit. The remaining fields batch i 2720,batch j 2722, and batch k 2724 are the individual batches directed tothe mobile units.

It should be understood that different classes of mobile units canfollow different desirable batch protocols, depending on the type ofservice, processing power, battery capacity, or other factors.

The individual batch protocol 2780 is shown in FIG. 27(C). The batchheader field 2726 is similar to the header fields discussed above forFIGS. 27(A) and (B). The batch header 2726 includes a list of particularmobile units to receive messages within the batch and includes timingoffsets indicating when such messages will be broadcast. Further, thebatch header 2726 includes data indicating a timing offset scheduling areverse channel interval in the system reverse interval, the zonalreverse interval, or the reverse contention interval, as appropriate.Again, this information allows the mobile unit to extend its batterylife because the mobile unit need only power up at the appropriate timeto receive or transmit the appropriate message. Further, it is preferredthat the reverse channel timing offset data be transmitted using errorcorrection codes to insure accurate receipt thereof by the mobile unit.Accurate receipt of the reverse channel timing offset data will preventunwanted or untimely transmissions by the mobile unit and insure that amobile unit may properly transmit a negative acknowledgment signal iffails to properly receive an unencoded message.

The individual message interval 2732 includes the individual messageintended for a particular mobile unit or units. The duration of eachmessage and number of messages within a batch may be varied by thenetwork operations center 600 and is traffic dependent.

Each mobile unit with transmit capability that has received a message inthe immediately previous systemwide forward interval 2704 or the zonalforward interval 2708 will have an appropriate time slot fortransmission scheduled in the systemwide response interval 2706, or thezonal reverse interval 2710, respectively. The timing circuit in themobile transceiver unit determines the assigned time slot fortransmission. For example, if the mobile unit simply intends to transmitan acknowledgment signal, which indicates that the mobile unit hasproperly received the message from the network, an 8 bit preamblefollowed by the address of that mobile unit need only be transmitted anda 3 bit acknowledgment. However, if a more extensive reply from themobile unit is required, additional data could be transferred duringthis time slot. In particular, long reverse messages could be scheduledin response to a request from the mobile unit sent during the contentioninterval 2712, as discussed hereafter.

Due to the low power transmit capability of the mobile transceiverunits, there is an increased likelihood of data transmission errors forreply signals. The extended Golay code for error protection may beutilized for reverse channel messages from mobile transceiver units tothe network.

The systemwide response interval 2706 and the zonal reverse interval2710 provide communication capability from the mobile transceiver unitsto the network (i.e. the reverse channel).

Still further, a preferred embodiment accommodates mobile terminals withextensive reverse message generation capabilities (e.g., a laptopcomputer connected to a radio transceiver) by allowing for contentionmessages that request extended reverse channel time for the transmissionof a long reverse message. The reverse contention interval 2712 islocated after the zonal reverse interval 2710 and provides forunscheduled messages from the mobile unit to the network. For example,the mobile transceiver unit could send a message to the network duringthe reverse contention interval 2712 indicating that the user no longerwishes to receive messages, thereby terminating service. Also, the usercould transmit a message to the network during the reverse contentioninterval 2712 indicating that the user now desires to reestablishservices and begin receiving messages from the network. Further, a"registration signal," which is discussed infra, could be transmittedduring the reverse contention interval 2712.

The reverse contention interval preferably utilizes a so-called "slottedALOHA" protocol, which allows the mobile unit to randomly select apredefined time slot within the contention interval to transmit amessage. A mobile station wanting to transmit will first divide thecontention interval into slots, preferably 5.33 ms in length, and thenchoose randomly any of them to start transmitting. The slotted ALOHAprotocol is preferred because of the low likelihood of data "collisions"(i.e. 2 or more mobile units transmitting during the same time slot).

I. Registration of the Mobile Unit

Because the network operations center 600 stores the location of eachmobile unit in the system in the user database 2100, it is preferredthat each mobile transceiver unit have the capability to "register" withthe network operations center 600 by sending a registration signal to abase receiver into the network to update the location data.

The mobile transceiver unit preferably registers by simply transmittingits identification number to a base receiver, which forwards this dataand data representing the location of the base receiver to the networkoperations center 600.

The mobile transceiver preferably registers upon crossing zonalboundaries to alert the network operation center that the mobiletransceiver has left one zone and entered another. For example, themobile unit could receive information from the nearest base transmitteridentifying which zone that base transmitter is assigned to at thebeginning of each communication cycle. Upon receipt of such informationfrom a base transmitter indicating that a nearby base transmitter isassigned to a new zone, the mobile transceiver then preferably transmitsa registration signal.

The mobile transceiver unit may also transmit a registration signal inother desirable instances. For example, if the mobile transceiver unithas moved away from the transmitter coverage areas of the network for aperiod of time, the mobile transceiver unit may preferably transmit aregistration signal upon returning to a coverage area. The display andstorage logic 1508 of the mobile transceiver unit preferably recognizesthat the unit has left the coverage area of the network upon failure toreceive data from a base transmitter in the network during the cycleheader time interval 2702, for example. The mobile unit may leave thecoverage area of a base transmitter of the network when the user takesthe unit out of the country, or enters the basement of a building, forexample.

The mobile unit may also preferably transmit a registration signal whenpower is restored to the mobile unit after having power removed, such asafter being turned off by the user. Of course, the power may be restoredto the unit by replacing or recharging a dead battery, which may alsocause transmission of a registration signal.

In general, the network must balance the need for frequent registrationsby the mobile transceiver units, and the desirable result of accuratelyknowing the location of each mobile unit, thereby preventing the needfor probe signals, with the undesirable overhead costs of too frequentregistration, which sacrifices data throughput by utilizing valuabletransmit time.

In the preferred embodiment, the central computer 2002 of the networkoperations center 600 can achieve desirable performance by implementingone or more algorithms to evaluate the need for registration by a mobileunit, and then appropriately controlling the registration performance ofthat mobile unit. If the central computer determines that registrationof a particular mobile unit is useful, then the mobile unit preferablyshould receive a message from the network to cause the mobile unit tosend registration signals at appropriate times. Conversely, if thecentral computer determines that the registration signals from themobile unit are too frequently not useful, the mobile unit preferablyshould receive a message from the network to cause the mobile unit notto transmit registration signals.

To implement this feature, the mobile transceiver unit furtherpreferably includes a registration flag (not shown) in the display andstorage logic section 1508. If the registration flag is set, the displayand storage logic section 1508 causes the mobile transceiver toautonomously send a registration signal to the network operations centeron a desired basis. If the registration flag is not set, the display andstorage logic section 1508 prevents any registration signals from beingsent. The registration flag may be set or removed upon command from thenetwork operations center by transmission of an appropriate signal froma base transmitter near the mobile unit. A variety of algorithms,possibly regarding individual users or groups of users, can be used todetermine whether or not the registration flag should be set. It shouldbe appreciated that the present invention provides two distinctalgorithms for implementing these registration concepts depending uponwhether the registration flag is set or not in the mobile unit (i.e. thestate of the mobile unit).

FIG. 28(A) shows a flow chart describing a preferred method 2800 forimplementing the registration concepts of the present invention whereinthe registration feature of the mobile unit is disabled. In step 2802,the network sends a message to disable the registration feature (i.e.set the registration flag to zero) of the mobile unit to disable themobile transceiver's capability to transmit a registration signal. Ascan be seen, step 2802 determines the initial state for the method setforth in FIG. 28(A).

In step 2804, the network stores the number of probe signals sent to themobile transceiver during a first period of time, and the number ofmessages successfully delivered to the mobile transceiver by the networkduring a second period of time. Preferably, the first and second timeintervals are identical. The traffic statistics database 2200 of thedatabase 2008 is preferably used to store the number of probe signalsand successful messages for each mobile unit. As explained hereinafter,these two statistics from the operation of the network are preferablyused to determine whether registration by the mobile unit is useful.

In step 2806, the stored number of probe signals and number of messagessuccessfully delivered is processed to evaluate a likelihood that aprobe signal will be required to be set by the network to locate themobile unit to deliver a message. The preferred embodiment of theinvention processes the stored number of probe signals and messagessuccessfully delivered in accordance with the method set forth in FIG.29(A).

Referring now to FIG. 29(A), therein is shown a series of substeps whichare preferably performed during the implementation of the processingstep 2804 shown in FIG. 28(A). In particular, steps 2902 and 2904 areevent driven and only proceed to the next: step after an input has beenreceived by the network. Step 2902 determines if the network sent aprobe signal to a lost mobile transceiver unit and if a reply to theprobe signal was received by a base receiver in the network. If thisevent occurs, a counter (not shown) is incremented by a value P by thecentral computer 2002.

In step 2904, if a message was successfully delivered to a mobiletransceiver, preferably including an acknowledgment signal return fromthe mobile transceiver to the network, the counter (not shown) in thecentral computer 2002 is decremented by a value D.

After the occurrence of either of the events tested for in step 2902 orstep 2904, the algorithm proceeds to step 2906. In step 2906, if thecounter value is greater than a predetermined value J, this indicatesthat the likelihood that a probe signal will be necessary to locate themobile transceiver is greater than a selected value.

As can be seen, the process of substeps in FIG. 29(A) balances thefrequency of probe signals sent to a particular unit against the numberof successfully delivered messages to that unit. If the system must senda large number of probe signals, it would be useful to enable theregistration feature by setting the registration flag on that mobileunit to enable the registration feature. In contrast, if many messageshave been successfully delivered without requiring a probe signal, it isunnecessary to enable the registration feature by setting theregistration flag.

In step 2808, a message is sent to the mobile unit to enable the mobiletransceiver's capability to transmit a registration signal if thecalculated likelihood in step 2804 exceeds a selected value. As can beseen, step 2808 preferably sets the registration flag in the mobiletransceiver unit.

FIG. 28(B) shows a flow chart describing a method 2810 for implementingthe registration concepts of the present invention wherein theregistration feature of the mobile unit is enabled. In step 2812, thenetwork sends a message to enable the registration feature (i.e. set theregistration flag to 1) of the mobile unit to enable the mobiletransceiver's capability to transmit a registration signal. As can beseen, step 2812 determines the initial state for the method set forth inFIG. 28(B).

In step 2814, the network stores the number of registration signalsreceived by the network during a first period of time, and the number ofmessages successfully delivered to the mobile transceiver by the networkduring a second period of time. Preferably, the first and second timeintervals are identical. The traffic statistics database 2200 of thedatabase 2008 is preferably used to store the number of registrationsignals and successful messages for each mobile unit. As explainedhereinafter, these two statistics from the operation of the network arepreferably used to determine whether the registration by the mobile unitis useful.

In step 2816, the stored number of registration signals and number ofmessages successfully delivered is processed to evaluate the likelihoodthat a registration signal will be received by a base receiver in thenetwork that will not be used by the network to determine a set of basetransmitters to be operated to transmit a message to the mobiletransceiver. The preferred embodiment of the invention processes thestored number of registration signals received and number of messagessuccessfully delivered in accordance with the method set forth in FIG.29(B).

Referring now to FIG. 29(B), therein is shown a series of substeps whichare preferably performed during the implementation of the processingstep 2814 shown in FIG. 28(B). In particular, steps 2912 and 2914 areevent driven and only proceed to the next step after an input has beenreceived by the network. Step 2912 determines if a registration signalwas received by a base receiver in the network. If so, a counter (notshown) in the central computer 2002 is incremented by a value A.

In step 2914, if a message was successfully delivered to a mobiletransceiver, preferably including an acknowledgment signal return fromthe mobile transceiver to the system, the counter (not shown) in thecentral computer 2002 is decremented by a value M.

It should be understood that the counter referred to with regard tosteps 2912 and 2914 is different then the counter referred to withregard to steps 2902 and 2904 since each counter is only necessary whenthe registration feature is enabled or disabled in the mobiletransceiver. However, the same physical or logical device may be used toimplement both counters.

After the occurrence of either events in the step 2912 or step 2914, thealgorithm proceeds to step 2916. In step 2916, the process determines ifthe counter value is greater than a predetermined value T. The value ofT can be varied to meet the needs of a particular network. When thecounter value exceeds T, it is indicated that the likelihood that aregistration signal from that mobile unit will not be used by thenetwork to determine a new set of base transmitters, and therefore theregistration status for that mobile unit needs to be changed to disablethe registration feature.

In other words, the process in FIG. 29(B) balances the frequency ofregistration signals sent by a particular unit against the number ofsuccessfully delivered messages to that unit. As can be seen, if themobile unit sends a large number of registration signals without thesystem using these registration signals, it would be useful to have theregistration feature on that mobile unit disabled. In contrast, if manymessages have been successfully delivered without too many registrationsignals being sent by the mobile unit, it is unnecessary for theregistration feature to be disabled.

In step 2818, a message is sent to the mobile unit to disable the mobiletransceiver's capability to transmit a registration signal if thecalculated likelihood in step 2814 exceeds a selected value. As can beseen, step 2818 may preferably remove the registration flag in themobile transceiver unit.

Of course, it should be understood that the variables P, D, and J usedin FIG. 29(A), and the variables A, M, and T used in FIG. 29(B) can beadjusted as desired to enhance system performance, as will be apparentto one of ordinary skill in the art. The counters can be implementedwith so-called "reflective boundaries" so that if a counter reaches aminimum value (e.g., zero), it will continuously reset to that minimumvalue when further decremented.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the systems and methods ofthe present invention without departing from the scope or spirit of theinvention.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A mobile transceiver unit for transmittingmessages to and receiving messages from a network comprising:input meansfor allowing a user to input a user message to the unit; transmittermeans for transmitting a radio frequency signal including the usermessage from the mobile unit to the network; receiver means forreceiving radio frequency signals having a message from the network;signal detector means for detecting a noise signal at approximately 400Hz generated by an aircraft; means for determining whether the detectednoise signal exceeds a predetermined threshold; and a circuit fordisabling the transmitter means upon the determining means determiningthat the detected noise signal exceeds the predetermined threshold,thereby preventing unwanted radio frequency transmission.
 2. The mobiletransceiver unit of claim 1, further comprising:a plurality of inputswitches allowing the user to transmit a signal from the mobiletransceiver unit.
 3. The mobile transceiver unit of claim 2, wherein theplurality of input switches allow the user to transmit a reply to areceived message.
 4. The mobile transceiver unit of claim 3, furthercomprising:a display for displaying the received message and displayinga reply message associated with each of the plurality of input switches.5. The mobile transceiver unit of claim 4, further comprising:means forgenerating and displaying the reply messages associated with each of theplurality of input switches based upon the contents of the receivedmessage.
 6. The mobile transceiver unit of claim 1, furthercomprising:means for demodulating the received radio frequency signalsfrom the network using a predetermined demodulation technique; and meansfor adaptively changing the demodulation technique to accommodatedifferent signal modulation formats.